Medium-frequency radars measure winds (between 55 and 100 km altitude). As part of their final year undergraduate project, the lead author used two radars in Antarctica to look at how the wind speed varied with the aim of identifying when the wind was too fast to be a real measurement. Instead, we discovered that the variance depends strongly on factors in the analysis technique rather than on natural features such as gravity waves, and that the Sun and geomagnetic activity play a role.